Global warming is the rise in the averagetemperature of Earths atmosphere and oceans since the late 19th century and its projected continuation. Since the early 20th century, Earths mean surface temperature has increased by about 0.8 °C (1.4 °F), with about two-thirds of the increase occurring since 1980.Warming of the climate system is unequivocal, and scientists are more than 90%certain that it is primarily caused by increasing concentrations of greenhouse gases producedby human activities such as the burning of fossil fuels and deforestation. These findings are
Climate model projections were summarized in the 2007 Fourth Assessment Report (AR4) by the Intergovernmental Panel on ClimateChange (IPCC). They indicated that during the 21stcentury the global surface temperature is likely to rise a further 1.1 to 2.9 °C (2 to 5.2 °F) for theirlowest emissions scenario and 2.4 to 6.4 °C (4.3 to 11.5 °F) for their highest. The ranges of these estimates arise from the use of models with
(According to AR4) warming and related changeswill vary from region to region around theglobe. The effects of an increase in globaltemperature include a rise in sea levels and achange in the amount and pattern of precipitation,as well a probable expansionof subtropical deserts.Warming is expected tobe strongest in the Arctic and would beassociated with the continuing retreat ofglaciers, permafrost and sea ice. Other likelyeffects of the warming include a more frequentoccurrence of extreme-weather eventsincluding heat waves, droughts and heavyrainfall, ocean acidification andspecies_risk_from_climate_change"extinctions due to shifting temperature regimes.
Proposed policy responses to global warming include mitigation by emissions reduction, adaptation to its effects, and possiblefuture geoengineering. Most countries are parties tothe United Nations Framework Convention on Climate Change (UNFCCC),] whose ultimate objective is to prevent dangerous anthropogenic (i.e., human-induced) climate change. Parties to the UNFCCC have adopted a range of policies designed to reduce greenhouse gas emissions100and to assist in adaptation to global warming.Parties to theUNFCCC have agreed that deep cuts in emissions are required, and that future global warming should be limited to below 2.0 °C (3.6 °F) relative to the pre- industrial level.Reports published in 2011 by the and the suggest that efforts as of the early 21st century to reduce emissions may be inadequate to
The Earths average surface temperature roseby 0.74±0.18 °C over the period 1906–2005. The rate ofwarming over the last half of that period was almostdouble that for the period as a whole (0.13±0.03 °C perdecade, versus 0.07±0.02 °C per decade). The urban heatisland effect is very small, estimated to account for lessthan 0.002 °C of warming per decade since1900. Temperatures in the lower troposphere haveincreased between 0.13 and 0.22 °C (0.22 and 0.4 °F) perdecade since 1979, according to satellite temperaturemeasurements. Climate proxies show the temperature tohave been relatively stable over the one or two thousandyears before 1850, with regionally varying fluctuationssuch as the Medieval Warm Period and theLittle Ice Age.
The warming that is evident in the instrumentaltemperature record is consistent with a wide range of observations, as documented by many independent scientific groups. Examples include sea level rise (water expands as it warms), widespread melting of snow and ice, increasedheat content of the oceans, increased humidity, and theearlier timing of spring events, e.g., the flowering of plants. The probability that these changes could have occurred by chance is virtually zero.
Recent estimates by NASAs Goddard Institute for Space Studies (GISS) and the National Climatic Data Center show that 2005 and 2010 tied for the planets warmest year since reliable, widespread instrumental measurements became available in the late 19th century, exceeding 1998 by a few hundredths of a degree. Estimates by the Climatic Research Unit (CRU) show 2005 as the second warmest year, behind 1998 with 2003 and 2010 tied for third warmestyear, however, "the error estimate for individual years ...is at least ten times larger than the differences between these three years." The World Meteorological Organization (WMO) statement on the status of the global climate in 2010 explains that, "The 2010 nominal value of +0.53 °C ranks just ahead of those of 2005 (+0.52 °C) and 1998 (+0.51 °C), although the differences between the three years are not statistically significant..."